Friday, February 27, 2009

The study of manual lateralization in chimpanzees is today a subject of renewed interest. Currently there is no consensus whether manual dominance does or does not exist at a species level, mainly for wild and semicaptivity populations. The aim of this study was to evaluate the manual laterality in a group of chimpanzees living in an intermediate setting (semicaptivity) through two tasks: one simple and unimanual (simple reaching) and the other complex and bimanual (tube task).

Charly at a tube-task session - Miquel Llorente / IPHES

In simple reaching the hand is used to gather food with precision, and the type of grip and the posture are all evaluated. The tube task evaluates the hand used to extract food from the tube and the method of extraction (digital or instrumental). Through the handedness index we observed that the individuals show clear and strong individual lateralization for both tasks, particularly for the tube task (100%), but also for simple reaching (86%). Anyway, we did not detect population preferences for any of the tasks. However, considering both tasks jointly (multiple evaluation) it has been possible to detect for the first time a skilled manual dominance at the population level in semicaptive chimpanzees.

As a conclusion, our chimpanzee sample displays right handedness at population level when considering multitask values. This is the first time that data on handedness are recorded at population level in a chimpanzee sample living in an intermediate and naturalistic setting. Former researches on hand laterality with the same sample gave negative results (Mosquera et al. 2007), but that study was focused in unimanual and spontaneous tasks. Therefore, results of the current work point to three factors as decisive to force the displaying of hand laterality in chimpanzees: bimanual coordination, tool use, and grip precision.